The bystander effect in photodynamic inactivation of cells

Treatment of MDCK II cells with the lipophilic photosensitizer tetra(3-hydr oxyphenyl)porphyrin and light was found to induce a rapid apoptotic respons e in a large fraction of the cells. Furthermore, the distribution of apopto tic cells in microcolonies of eight cells was found to be different from th e binomial distribution, indicating that the cells are not inactivated inde pendently, but that a bystander effect is involved in cell killing by photo dynamic treatment, The observation of a bystander effect disagrees with the common view that cells are inactivated only by direct damage and indicates that communication between cells in a colony plays a role in photosensitiz ed induction of apoptosis. The degree of bystander effect was higher for ce lls dying by necrosis than for cell dying by apoptosis. (C) 2000 Elsevier S cience B.V. All rights reserved.